Micropeptin T-20, a novel glyceric acid 3-O-phosphate and 3-amino-6-hydrooxy-2-piperidonecontaining cyclic depsipeptide, was isolated from a cyanobacterium Microcystis aeruginosa. The structure was identified as 1 by 2D NMR and chemical degradation analyses. Micropeptin T-20 inhibited chymotrypsin.